An image formation in an image forming apparatus such as an electrophotographic copier or a printer is carried out as described below. Firstly, an area on the surface of an electrophotographic photosensitive body where a latent image is to be formed is uniformly charged with a charger. Next, the latent-image forming region is irradiated with an exposure light to remove charges on the electrophotographic photosensitive body, which is charged with the charger, according to a desired image pattern, whereby an electrostatic latent image is formed. Then, the electrostatic latent image is developed with toner to form a toner image, and then, the toner image is transferred and fixed onto a recording sheet. Next, the toner remaining on the surface of the electrophotographic photosensitive body is removed. Subsequently, the electrophotographic photosensitive body is irradiated with an eliminating light to eliminate the charges thereon. When the above-mentioned processes are repeated, an image is formed on a recording sheet.
In the image forming apparatus described above, an irradiation area of a laser beam used as the exposure light is increased as the angle of incidence of the beam on the electrophotographic photosensitive body is increased. Therefore, in the image formation described above, the light intensity changes according to the incidence angle of the laser beam. The conductivity of a photosensitive layer in the electrophotographic photosensitive body becomes high as the intensity of the light incident on the photosensitive layer is increased. Therefore, when the laser beam is scanned on the electrophotographic photosensitive body that is uniformly charged, the static elimination amount varies in the scanning direction of the laser beam, and further, the charging amount differs in the axial direction of the electrophotographic photosensitive body. When the charging amount differs as described above, the amount of toner, which is deposited with electrostatic force between the charged charges and the toner, varies, which appears as a difference (unevenness) in a color density of an image formed onto a recording sheet.
From the above, in the image forming apparatus described above, non-uniform image density might be produced in the scanning direction of the laser beam.
In view of this, an image forming apparatus has been developed in which a substantial light quantity is changed by the control of on/off time of the laser beam so as to reduce non-uniform image density. The apparatus described above is disclosed, for example, in Patent Document 1. In the image forming apparatus disclosed in Patent Document 1, in order to reduce a variation between a region where density becomes relatively low (hereinafter referred to as a “high static eliminating region”) and a region where density becomes relatively high (hereinafter referred to as a “low static eliminating region”), an area of a static eliminating region in the high static eliminating region is made greater than an area of a static eliminating region in the low static eliminating region, wherein non-uniform image density is reduced by employing a human's visual characteristic.    Patent Document 1: Japanese Unexamined Patent Publication No. 05-011209